In some large client sites, we see many queries from the session handling code in drupal, mostly from the sess_write() function in includs/session.inc.
Examples of creating a new session:

INSERT INTO sessions (sid, uid, cache, hostname, session, timestamp) VALUES ('744e888350d5c759f067d58380f6874e', 0, 0, '10.1.1.1', '', 1243567441)

And examples for updating a session:

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Zend have had their own proprietary PHP server product for quite some time now. This week, they announced their Zend Server Community Edition, with the intention to disrupt Java, and grow PHP specially in the current economic conditions.

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One of the most useful features in Drupal is its cron hook. It allows modules to execute stuff at regular intervals when cron.php is run.
This is used for many things, such as indexing new content that was added to the site, cleaning up old watchdog entries and many other things.
In many cases, though cron hooks implemented by modules can cause added burden to a site. This is specially true if you run cron too frequently or too infrequently.
An example for a client came up recently. They are using xmlsitemap, like many sites do.

When tuning sites for clients, we often see the usual symptoms and causes of why a site is slow, and occasionally we find an unusual reason.
For one client, after we diagnosed the main reason for a site being slow (bottleneck between database server and web server), they continued to experience slowdowns.
This was getting annoying as it happened several times a day, and their customers were complaining about the slowdown on a daily basis.

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